Physical and Spectral Characteristics of the T8 and Later Type Dwarfs

摘要: We use newly observed and published near-infrared spectra, together with synthetic spectra obtained from model atmospheres, to derive physical properties of three the latest type T dwarfs. A new R ≈ 1700 spectrum T7.5 dwarf HD 3651B, existing data, allows a detailed comparison well-studied very similar Gl 570D. find that 3651B has both higher gravity metallicity than 570D, best-fit atmospheric parameters Teff = 820-830 K, log g 5.4-5.5, [m/H] +0.2, Kzz 104 cm2 s-1. Its age is 8-12 Gyr, its implied mass 60-70 MJ. perform analysis T8 dwarfs 2MASS J09393548-2448279 J11145133-2618235 using comparing them T8, J04151954-0935066. these two have effectively same as reference dwarf, or slightly gravities, but lower metallicities. The derived are 725-775 K -0.3; 5.3 - 5.45 for 5.0 J11145133-261823. ~10 Gyr 60 MJ J09393548-2448279, ~5 40 serious limitation such analyses incompleteness line lists transitions CH4 NH3 at λ ≤ 1.7 μm, which also needed synthesizing later, cooler, Y type. Spectra Saturn Jupiter, laboratory gas, suggest features in J bands may be useful indicators next spectral type, not H bands, previously thought. However, until cooler objects found, improve, large uncertainties remain, abundance likely significantly below chemical equilibrium value. Moreover, inclusion opacities our models predicts band shapes discrepant data. It possible class will extended temperatures around 400 when water clouds condense atmosphere dramatically change energy distribution brown dwarf.